The present invention relates to wireless communications systems and, in particular, to methods and apparatus of improving the performance of a CDMA communications system in which a base station communicates with moving mobile units.
The widespread deployment of wireless cellular communications systems can be attributed to an explosion in the public""s demand for high user mobility, coupled with advances in technology allowing multiple users to share a common frequency band virtually without interfering with one another. One widely used multiple-access technique, known as code-division multiple-access or CDMA, has been shown to exhibit superior performance with respect to many other multiple-access techniques in terms of countering the effects of multipath fading and mutual interference.
Many implementations of CDMA in use today are based on TIA/EIA standard IS-95, which is incorporated by reference herein. Some basic principles of IS-95 CDMA are explained in greater detail in several U.S. Patents, among which U.S. Pat. Nos. 5,109,390 and 5,103,459 (both to Gilhousen et al.) are particularly noteworthy and are incorporated by reference herein. In the years to come it is expected that a large number of industry leaders will adopt an improved flavour of CDMA, known to practitioners in the field as third generation (or xe2x80x9c3Gxe2x80x9d) CDMA. In accordance with various proposals being made by numerous groups working to standardize it, 3G CDMA would be similar to IS-95 CDMA but would provide additional desirable features such as a reverse-link pilot channel, data transmission rate negotiation and support for mobile unit speeds up to 500 km/h. An example of a 3G CDMA proposal is an International Telecommunications Union (ITU) document entitled xe2x80x9ccdma2000 RTT Candidate Submission to US TG8/1xe2x80x9d, which is incorporated by reference herein.
It is known that motion of a mobile unit may cause degradation in the performance of a CDMA receiver, both at the base station and at the mobile unit. For example, under multipath conditions, motion of the mobile unit relative to the base station and numerous signal scattering objects will cause the phase and strength of the corresponding received multipath signals to vary with time, resulting in a Doppler spread of the original signal. This makes it difficult for the receiver to estimate and/or track the channel phase and estimate the channel state information (CSI), particularly at high speeds.
Other effects of the speed of the mobile unit include a reduced lifetime of the individual multipath paths at high speeds (requiring fast finger acquisition), quickly varying multipath delays at high speeds (making tracking of the corresponding multipath paths more difficult for the receiver), as well as longer error bursts at low speeds (rendering error randomization more difficult for the deinterleaver at the receiver).
The approach taken by both IS-95 CDMA and the various 3G proposals in order to improve the immunity of CDMA signals to multipath fading and interference depends on the speed range which is to be supported. For instance, a closed loop power control scheme can be used to improve performance of a CDMA receiver at low speeds, while interleaving and coding tends to improve performance at high speeds. Because many effects of the Doppler spread are more pronounced at high speeds, conventional CDMA receivers are usually designed to achieve a certain performance measure at a predetermined maximum speed, e.g., 200 km/h for a receiver operating in the 1.9 GHz band.
Once the maximum supported speed is chosen, many other system parameters, such as the details of the finger assignment algorithm and the length of various accumulators in the receiver, are then optimized to meet the specific requirements for the maximum supported speed. However, the selection of these system parameters as a function of the xe2x80x9cworst case scenarioxe2x80x9d usually results in impaired performance at speeds both lower and higher than the speed chosen for optimization. This becomes a more severe issue as 3G mobile communications systems target a wider range of mobile unit speeds, up to a maximum of approximately 500 km/h.
Degraded receiver performance then leads to other deleterious effects, such as a reduction in the system capacity and a shorter life span for the mobile unit battery.
It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art.
According to one broad aspect, the invention is intended for use in a communications system wherein a base station communicates CDMA signals with a plurality of mobile units and can be summarized broadly as a method of processing the CDMA signals communicated between the base station and each mobile unit as a function of an estimate of the speed of the respective mobile unit.
According to another broad aspect, the invention can be summarized as a communications device comprising processing fingers which adapt their processing parameters as a function of an estimate of the speed of the mobile unit. The invention can be summarized according to further broad aspects as the individual components of a processing finger in the communications device, such as the PN code despreader, the phase estimator and the CSI estimator, each of which adapts its processing parameters as a function of an estimate of the speed of the mobile unit.
In a communications system wherein a base station communicates CDMA signals with a plurality of mobile units, the invention may be summarized broadly as a method of processing the CDMA signals communicated between the base station and each mobile unit as a function of an estimate of the speed of the respective mobile unit.
The invention may summarized according to another broad aspect as a method of varing the transmission rate of signals communicated between the base station and each mobile unit as a function of an estimate of the speed of the respective mobile unit.
In a communications system wherein a base station communicates CDMA signals with a plurality of mobile units and wherein the signals communicated in either direction between the base station and each mobile unit have a respective power level, the invention may be summarized according to a further broad aspect as a method of varying the power level of the signals as a function of an estimate of the speed of the respective mobile unit.
At a system level, involving both base station and mobile station, the invention can be summarized according to a further broad aspect as a method of adjusting, individually or in combination, the transmitted power, the relative power of the pilot channel, the data transmission rate and the interleaver/deinterleaver length as a function of an estimate of each mobile unit""s speed.
Moreover, the invention can be summarized according to still another broad aspect as a speed estimator for estimating the speed of a mobile unit. Different speed estimators are used, depending on whether or not a pilot channel is present and whether or not orthogonal modulation has been employed at the transmitter.
A speed estimator in accordance with the invention can be located at the base station but can be used for adapting parameters at the base station and/or at the mobile unit. Similarly, a speed estimator can be located at a mobile unit, but can be used for adapting parameters at the base station or at the mobile unit itself.